An additional subject matter of the invention is a process for preparing transistor channels, in particular in electronics, electron-accepting materials, in particular in photovoltaics, nonlinear infrared photon emitters or absorbers, current-conducting electrodes, flexible transparent electrodes, antistatic coatings, chemical detectors or solar cells, characterized in that it employs the process for the selective functionalization of metallic single-walled carbon nanotubes according to the invention.
Carbon nanotubes are a promising material simultaneously for their mechanical, electronic and optical qualities. While industrial uses are beginning to see the light in the field of mechanical reinforcing by multi-walled carbon nanotubes (MWNTs), applications in electronics and in optics are slower to arrive. This is because, in these fields, it is the single-walled carbon nanotubes (SWNTs) which show the most novel and the most effective properties.
Unfortunately, single-walled carbon nanotubes (SWNTs) can, depending on the chirality, that is to say the geometry of the winding of the carbon plane, either be metallic (m-SWNTs) or semiconducting (sc-SWNTs). Whatever the method of synthesis, metallic SWNTs and semiconducting SWNTs are generally synthesized in the form of a mixture of the two types.
Metallic and semiconducting SWNTs are both advantageous but for different applications. When metallic and semiconducting SWNTs are in a mixture, the presence of one type is often detrimental to the use of the other.
For example, semiconducting SWNTs can be used as transistor channels, in particular in electronics, or as electron-accepting materials, in particular in photovoltaics; the presence of metallic SWNTs among the semiconducting SWNTs then brings about short circuits. The semiconducting SWNTs can also be used as nonlinear infrared photon emitters in photonics; the metallic SWNTs in contact with the semiconducting SWNTs bring about the extinction of the luminescence.
In the same way, metallic SWNTs can, for example, be used as materials for current-conducting wires, vias or electrodes; the semiconducting SWNTs present among the metallic SWNTs bring about high instability of the conductivity.
The separation of the metallic SWNTs from the semiconducting SWNTs is thus a high requirement identified from the time of their discovery and constitutes the main blocking point for their industrial use. The separation of metallic SWNTs from semiconducting SWNTs has thus become, since the 2000s, a highly competitive and particularly active subject.
Various methods for differentiating, enriching or separating the SWNTs have already been described in the literature.
One of these methods consists in carrying out the selective coupling of the metallic SWNTs with diazonium (Strano M S. et al., Science, 2003, 301, pp. 1519-1522) with the aim of separating the semiconducting SWNTs from the metallic SWNTs (Dyke C A. et al., Journal of the American Chemical Society, 2005, 127, pp. 4497-4509). As the selectivity of this reaction is low, it does not make it possible to obtain either good separation of the starting material or a starting material of good quality. This is because the selective chemical differentiation of the metallic SWNTs is difficult given that the reactivity of the SWNTs does not depend only on the electronic type of the SWNTs but also in large part on their diameter. Thus, the semiconducting SWNTs having a small diameter (diameter generally of less than 1.1 nm) thus have a tendency to react in the same way as the metallic SWNTs having a large diameter (diameter generally of greater than 1.1 nm), thus making it difficult to separate them.
Other teams have also used the reaction of diazoniums with SWNTs to differentiate and separate metallic SWNTs from semiconducting SWNTs. However, the selectivity of the reaction is rarely given by the authors and it is not always possible to determine it from the published data. In the reactions indicated below, the selectivity was roughly estimated from the published tables and spectra.
The group of J. Tour is a pioneer in the reaction of diazoniums with SWNTs. The studies relate to HiPco® SWNTs, which are SWNTs having small diameters, that is to say a diameter of less than or equal to 1.1 nm, in particular a diameter of between 0.9 and 1.1 nm, which are dissolved in aqueous solutions of SDS (sodium lauryl sulfate or sodium dodecyl sulfate). From the published tables and spectra, the selectivity with regard to metallic SWNTs can be estimated at approximately 7 (Doyle C D. et al., Journal of the American Chemical Society, 2008, 130, pp. 6795-6800). In other words, the speed of reaction of the metallic SWNTs is 7 times greater than the speed of reaction of the semiconducting SWNTs.
The group of M. Strano has been working on this reaction for 8 years. All the studies relate to HiPco® SWNTs dissolved in an aqueous solution of SDS, an anionic surfactant, sometimes compared with a neutral surfactant, such as Triton. The selectivity with regard to metallic SWNTs can be estimated at approximately 6 in the best cases, that is to say that the speed of reaction of the metallic SWNTs is approximately 6 times greater than the speed of reaction of the semiconducting SWNTs (Nair N. et al., Journal of the American Chemical Society, 2007, 129, pp. 3946-3954).
Wang and Shim have published a study on the selective functionalization of metallic SWNTs by diazoniums by CVD (chemical vapor deposition). The reactivity of individual SWNTs is measured and compared. The selectivity can be estimated at approximately 5, that is to say that the speed of reaction of the metallic SWNTs is 5 times greater than the speed of reaction of the semiconducting SWNTs (Wang C J. et al., Journal of the American Chemical Society, 2005, 127, pp. 11460-11468).
Ghosh and Rao have used the reaction of diazoniums with SWNTs for a separation of metallic SWNTs from semiconducting SWNTs. They are SWNTs obtained by electric arc (having a large diameter, that is to say a diameter of greater than 1.1 nm, in particular a diameter of between 1.2 and 2 nm, limits included) dissolved in SDS. The selectivity is not accessible. The differentiated SWNTs are separated by ultracentrifuging over a density gradient. The nanotubes are subsequently annealed (Ghosh S. et al., Nano Research, 2009, 2, pp. 183-191).
Lee et al. have recently used the reaction of diazoniums with SWNTs to suppress the conductivity of the metallic SWNTs and to use the unseparated mixture as source of semiconducting SWNTs for applications in electronics. They are CoMocat® SWNTs (having a small diameter, that is to say a diameter of between 0.7 and 0.9 nm) dissolved in an aqueous SDS solution. The selectivity with regard to the metallic SWNTs can be estimated, from the spectra, at approximately 3, that is to say that the speed of reaction of the metallic SWNTs is approximately 3 times greater than the speed of reaction of the semiconducting SWNTs (Lee C W. et al., Advanced Materials, 2010, 22, p. 1278).
It should be noted that, for all these methods, whether they are based on covalent coupling or complexing, the selectivity and consequently the separation are better when SWNTs having small diameters are concerned, that is to say a diameter of less than or equal to 1.1 nm (such as CoMocat or HiPco). If SWNTs having greater diameters are concerned, that is to say a diameter of greater than 1.1 nm, in particular a diameter of between 1.2 and 2 nm, limits included, such as the SWNTs obtained by electric arc, by laser or by CVD (chemical vapor deposition), the selectivity decreases. This is because it has been found that the physical and chemical properties are less marked when the diameter of the SWNTs increases; in particular, their reactivity has a tendency to decrease. Due to their better contact with the metal electrodes, semiconducting SWNTs having greater diameters are preferred for electronic devices (Zhang L et al., Journal of the American Chemical Society, 2008, 130, pp. 2686-2691).
Furthermore, during the reaction of diazoniums, in particular the aryldiazoniums, with the SWNTs, said aryldiazoniums prove to be selective toward metallic SWNTs but not specifically. In other words, the reaction does not make it possible to functionalize solely the metallic SWNTs and the diazoniums also react with the semiconducting SWNTs with a lower but not insignificant speed.
Application WO 2010089395 describes a process for the separation of SWNTs based on the reaction of a diazonium derivative, the diazoester, with the SWNTs in order to differentiate and separate the metallic SWNTs from the semiconducting SWNTs. This results in a covalent functionalization of the metallic SWNTs which is approximately 10 times greater than the functionalization of the semiconducting SWNTs. As the coupling suppresses the conductivity of the SWNTs, the metallic SWNTs can no longer produce a short circuit. The material can thus be used directly for applications in electronics, as recently demonstrated (Schmidt G. et al., Chemistry European Journal, 2011, 17, pp. 1415-1418). Once differentiated, the metallic SWNTs and the semiconducting SWNTs are separated from one another and then heat treated at 400° C. (Cabana J. et al., Journal of the American Chemical Society, 2007, 129, p. 2244) for more demanding applications, such as photonics. Despite a greater selectivity of this method in comparison with the methods already known, the separation of the SWNTs having large diameters as defined above remains difficult.
A complexing/adsorption method has recently been described in which the SWNTs, dissolved in an aqueous solution of SDS (sodium lauryl sulfate or sodium dodecyl sulfate), are separated by agarose gel chromatography (Tanaka T et al., Nano Letters, 2009, 9, pp. 1497-1500, and Moshammer K. et al., Nano Research, 2009, pp. 599-606). However, like the majority of separation methods, the separation is not perfect and the semiconducting SWNTs obtained still comprise a small fraction of highly conductive metallic SWNTs capable of producing short circuits in the electronic devices, for example.
Among all the methods for differentiating, enriching or separating SWNTs described in the literature, to date only the process for separation by ultracentrifuging over a density gradient has resulted in the commercialization of semiconducting SWNTs and metallic SWNTs in separated form (Arnold M S. et al., Nature Nanotechnology, 2006, 1, pp. 60-65). The semiconducting SWNTs and the metallic SWNTs separated according to this process are sold by Nanointegris at a price of $150/mg for a purity of 90% and $800/mg for a purity of 99%. Despite the high degree of purity advertized, 99% pure semiconducting SWNTs are not easy to use. For example, in a transistor channel using numerous SWNTs in parallel in order to bring about a fall in the overall resistance (approximately 6 kΩ by a carbon nanotube), a short circuit due to the presence of just one metallic single-walled carbon nanotube remains highly probable.
There thus remains a real need for a process which makes possible the differentiation of metallic SWNTs with respect to semiconducting SWNTs by a selective functionalization of said metallic SWNTs which overcomes the disadvantages of the prior art.
In particular, there exists a real need for a process which makes possible the functionalization of the metallic SWNTs, a significantly improved selectivity with respect to that described for the processes of the prior art, this being independently of the diameter of the SWNTs.
In addition, there exists a real need for a process which makes possible the functionalization of the SWNTs which can be carried out industrially.